Introduction
The International Telegraph and Telephone Consultative Committee (CCITT), an international communications standards organization, has been developing a standard for 16 kb/s speech coding and decoding for universal applications. The standardization process included the issuance by the CCITT of a document entitled "Terms of Reference" prepared by the ad hoc group on 16 kbit/s speech coding (Annex 1 to question 21/XV), Jun. 1988. The evaluation of candidate systems seeking to qualify as the standard system has thus far been divided into two phases, referred to as Phase 1 and Phase 2.
Presently, the candidate being consider for the standard is Low-Delay Code Excited Linear Predictive Coding (hereinafter, LD-CELP) described in substantial part in the incorporated application Ser. No. 07/298451. Aspects of this coder are also described in J-H Chen, "A robust low-delay CELP speech coder at 16 kbit/s," Proc. GLOBECOM, pp. 1237-1241 (Nov. 1989); J-H Chen, "High-quality 16 kb/s speech coding with a one-way delay less than 2 ms," Proc. ICASSP, pp. 453-456 (Apr. 1990); J-H Chen, M. J. Melchner, R. V. Cox and D. O. Bowker, "Real-time implementation of a 16 kb/s low-delay CELP speech coder," Proc. ICASSP. pp. 181-184 (Apr. 1990); all of which papers are hereby incorporated herein by reference as if set forth in their entirety. The patent application Ser. No. 07/298,451 and the cited papers incorporated by reference describe aspects of the LD-CELP system as evaluated in Phase 1. Accordingly, the system described in these papers and the application Ser. No. 07/298,451 will be referred to generally as the Phase 1 System.
A document further describing the LD-CELP candidate standard system was presented in a document entitled "Draft Recommendation on 16 kbit/s Voice Coding," submitted to the CCITT Study Group XV in its meeting in Geneva, Switzerland during Nov. 11-22, 1991 (hereinafter, "Draft Recommendation"), which document is incorporated herein by reference in its entirety. For convenience, and subject to deletion as may appear desirable, part or all of the Draft Recommendation is also attached to this application as Appendix 1. The system described in the Draft Recommendation has been evaluated during Phase 2 of the CCITT1 standardization process, and will accordingly be referred to as the Phase 2 System. Other aspects of the Phase 2 System are also described in a document entitled "A fixed-point Architecture for the 16 kb/s LD-CELP Algorithm" (hereinafter, "Architecture Document") submitted by the assignee of the present application to a meeting of Study Group XV of the CCITT held in Geneva, Switzerland on Feb. 18 through Mar. 1, 1991. The Architecture Document is hereby incorporated by reference as if set forth in its entirety herein and a copy of that document is attached to this application for convenience as Appendix 2. Also incorporated by reference as descriptive of the Phase 2 System and J. H. Chen, Y. C. Lin, and R. V. Cox, "A fixed point 16 kb/s LD-CELP Algorithm," Proc. ICASSP, pp. 21-24, (May 1991).
Tandeming
One requirement set by the CCITT involved performance when a series of encodings and decodings of input information occurred in the course of communicating from an originating location to a terminating location. Each of the individual encodings and decodings is associated with a point-to-point communication, while the concatenation of such point-to-point communications is referred to as "tandeming." CCITT specified performance requirements for both point-to-point performance and for three asynchronous tandems, i.e., tandeming of three encodings and decodings.
Tandem encodings of higher bit-rate coders such as 64 kb/s G.711 PCM and 32 kb/s G.721 ADPCM have been studied in detail over the years. The objective signaI-to-noise ratio (SNR) of these coders can be predicted by a simple model: the SNR drops 3 dB per doubling of the number of tandems. The assumption of this model is that the coding noise of each coding stage is uncorrelated with the coding noise of other coding stages. Under this assumption, if the number of tandems doubles, the noise power also doubles, and therefore the SNR drops by 3 dB. This model also predicts that improvements in the single encoding SNR of a coder do not change the relative amount that the SNR declines with successive encodings.
In tandeming experiments on the Phase 1 system, it was found that the SNR of 16 kb/s LD-CELP followed the -3 dB per doubling model quite well. Regardless of improvements to the SNR for a single encoding, the SNR always dropped by about 3 dB after 2 asynchronous encodings and by about 4.8 dB after 3 encodings.
The LD-CELP coder in the Phase 1 system (hereinafter, the "Phase 1 coder") had been carefully optimized for a single encoding under the delay and robustness constraints imposed by the CCITT. Thus improvements in the single encoding process SNR by a significant amount (even by only 0.5 dB) proved quite difficult. Although the single encoding speech quality of LD-CELP was quite good, after 3 asynchronous tandems, the coding noise floor increased by about 4.8 dB, resulting in relatively noisy speech. Even with an improvement of the single encoding SNR by 0.5 dB, the improvement would not be tripled after 3 encodings. The noise floor after 3 encodings would only be lowered by 0.5 dB, an insufficient improvement for some purposes.
Thus, in some respects, the so-called "Phase 1" system described in the above- incorporated application Ser. No. 07/298451 and incorporated papers, other than the Draft Recommendation, operated with degraded performance under tandeming conditions.
So-called postfilters have been used in the signal processing arts to improve the perceived quality of received signals. See, for example, U.S. Pat. No. 4,726,037 by N. S. Jayant on Feb. 16, 1988 and U.S. Pat. No. 4,617,676 issued Oct. 14, 1986 to N. S. Jayant and V. Ramamoorthy. Both of these patents are assigned to the assignee of the present application. While postfilters have been useful in some context, it has been the prevailing view that such techniques would not be useful in a Phase 1 System.